1. Technical Field
This invention relates to a receiver for receiving optical signals from an optical communications network for transmitting coded, multiplexed optical signals. The receiver includes means for detecting the optical signals to be received and processing means to compensate the noise present in the detected optical signals by combining the detected optical signals with compensation signals.
2. Discussion of Related Art
Such a receiver is known, for example, from the Proceedings of the Thirteenth Annual Conference on European Fiber Optic Communications and Networks, Brighton, England, 1995, pages 178 to 181 authorized by L. Mxc3x6ller. A receiver for a CDMA system is described which includes a periodic filter for detecting the optical signals to be received. The filter passes the detected optical signals to a photodiode, where they are converted from optical to electrical form. Also provided is an optical coupler which removes part of the optical signals transmitted through the CMDA network and routes these signals to a further photodiode. The removed signals serve as compensation signals which, after being converted from optical to electrical form, are first attenuated and then applied to the negative input of a differential amplifier. The positive input of the differential amplifier receives the detected signals from the filter converted to electrical form. The compensation of noise in the detected signals is achieved by comparing the detected signals with the compensation signals. A disadvantage is that the compensation signals used for the comparison include a part of the optical signals to be received, which is subtracted from the detected signal during the comparison, whereby the degree of compensation is deteriorated. The compensation method is only suitable for a CDMA system comprising a plurality of optical transmitters and receivers, since in such a system, the effect of the impairment of the compensation caused by the subtraction of the part of the signal to be received is reduced. Thus, the smaller the finesse of the periodic filter, the less efficient the compensation method will be; the finesse is explained in the above-mentioned article and is inversely proportional to the half-power bandwidth of the passband.
It is therefore an object of the invention to provide a receiver which uses an improved compensation method and is suitable for systems with as little as two transmitters.
According to the invention, this object is attained by a receiver for receiving optical signals from an optical communications network for transmitting coded, multiplexed optical signals, comprising a means for detecting the optical signals to be received and processing means for compensating noise present in the detected optical signals by combining the detected optical signals with compensation signals. The receiver is characterized in that it comprises a means adapted to pass optical signals to be received and reflect optical signals not to be received, and that a further means is provided for deriving the compensation signals from at least a part of the reflected optical signals and then applying them to the processing means. The receiver according to the invention has the advantage that the signal to be received, in particular, is not contained, or contained only minimally, in the compensation signal, so that it will not affect, or affect only insignificantly, the degree of compensation when being combined with the detected signal. Another advantage is that, in one embodiment, a test signal is additionally introduced into one of the signal branches in order to achieve even more selective compensation. These and other objects, features and advantages of the present invention will become more apparent in light of the detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing.